Magnetic resonance imaging (MRI) is primarily used in medical imaging to visualize anatomical structure of a patient's body. MRI technology can provide detailed images of the body in any plane. MRI has the ability to show soft tissue contrasts, which makes MRI scans especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging. MRI scans use a powerful magnetic field to align the magnetization of hydrogen atoms in the body. Radio waves are used to systematically alter the alignment of such magnetization, thereby causing the hydrogen atoms to produce a rotating magnetic field detectable by the MRI system. The resulting signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image or map of functional processes in a patient's body. A PET system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a non-metabolically or metabolically active molecule. As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters, the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions, which are then detected when they reach a scintillator material in a scanning device of the PET system. Images of metabolic activity and other physiological and biochemical events in space are then reconstructed by computer analysis.
The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other. Thus, it is possible to localize the source of the positron annihilation event along a straight line of coincidence (also referred to as a line of response (LOR)), and then an image reconstruction can be performed using coincidence statistics. For example, using statistics collected from a plurality of coincidence events, a set of simultaneous equations for a total activity of each parcel or bit of tissue (also called a voxel) along many LORs can be solved by a number of techniques, and thus a map of radio-activities as a function of location for parcels or bits of tissue can be constructed and plotted. The resulting map shows the tissues in which the molecular probe has become concentrated, and the resulting map can be interpreted by a physician and used for patient diagnosis and treatment. The parcels or bits of tissue can be quite small since the gamma ray event energy is large. This is one of the advantages of PET—a high signal to background ratio.
PET scans are commonly read alongside CT scans, in an attempt to produce a combination image by “co-registration” that gives the physician both anatomic and metabolic information about the patient's body. It is widely accepted that co-registration of anatomical information improves the diagnostic value of functional imaging, as can be seen in the success of hybrid scanners using PET and CT imaging. Recently, hybrid scanners using PET and MRI imaging have become available. The combination of PET and MRI may offer advantages, such as higher soft tissue contrast in the MRI anatomical images, real simultaneous acquisition, and minimum radiation exposure to the patient.
In general, a phantom is used to calibrate and/or verify the accuracy of nuclear medical imaging devices such as PET scanners or MR scanners. In essence, a phantom contains positron emitting activity in a known shape and uniform distribution of radiation activity throughout the body. Thus, by imaging the phantom with its known geometry and radiation distribution, the accuracy of the software used to assemble the various tomographic slices acquired by the imaging apparatus into three-dimensional representations of a patient's region of interest can be assessed and, if necessary, the various apparatus settings can be adjusted.
More specifically, the purpose of the alignment phantom is to measure the orientation and offset of the PET field of view with respect to the MR field of view.
Recently, a fully integrated magnetic resonance/PET scanner has been developed, which integrated scanner allows for simultaneous MR and PET imaging (see, e.g., U.S. Pat. No. 8,073,525, to Ralf Ladebeck et al. and incorporated herein by reference). Such a scanner requires a dedicated phantom which, in addition to facilitating quality control assessment of the PET scanning functionality of the machine, facilitates a determination of the extent to which operating the MR functionality of the machine influences the PET signals acquired by the machine.
A conventional phantom will not work for this purpose because the MR coil of the scanner has to experience a certain load in order to dampen the resonance peak sufficiently, even when maximum RF power is applied to the coil. Otherwise, if the MR coil is not loaded when maximum RF power is applied, too much current will be developed in the MR resonance circuit, and that current could potentially be large enough to destroy the MR coil itself.
Thus a need exists for a hybrid phantom that can accommodate a PET scanner, a MR scanner and/or a MR/PET scanner and overcome the problems in the prior art.